1. Field of the Invention
The present invention relates to an apparatus for a friction stir and a friction stir processing.
2. Description of Related Art
As a method for butt joining members, a friction stir welding (FSW) method is known. (Refer to Document 1 (Published Japanese Translation of PCT Application No. Hei 07-505090), for example.) The members are joined together in solid state by pressing a rotating tool against a seam so that frictional heat produced between the tool and the members is utilized to soften the members in proximity to the tool; and the tool is rotated and moved along the seam between the members. One application of the friction stir welding, is modification treatments using the friction stir welding process, including the modification of an aluminum cast, and the development of super plasticity by microstructure refining of wrought aluminum, composition carried out by dispersing another substance, such as powder, into a member, and the like. (Refer to Document 2 (Japanese Patent No. 3229556), Document 3 (Japanese Patent No. 3346380), and Document 4 (Japanese Patent Laid-open No. 2007-100129), for example.)
An example of the application of modification processing utilizing a friction stir is modification processing in proximity to a ring groove for a piston ring in a cylindrical piston used in internal combustion engines, such as gasoline engines. For the pistons of gasoline engines and the like, a cast of aluminum, aluminum alloy, or magnesium alloy (hereafter, referred to as “aluminum cast or the like”) is widely used. The pistons are provided on the circumferential surface thereof with a groove for fitting the piston ring and the surface of this groove is subjected to alumite treatment (anodic coating for aluminum and aluminum alloy) for ensuring wear resistance.
The smoothness of an alumite layer (coating) formed by alumite treatment is an important parameter that determines the sealability of the piston ring. In the case of the groove for fitting the piston ring, for example, an alumite layer inferior in smoothness causes compressed gas to escape from the combustion chamber to the crankcase, that is, it causes so-called blow-by. To cope with this, the following measure is taken: the area where this alumite layer is to be formed is modified by the friction stir to homogenize the metal structure of the substrate and then alumite treatment is carried out.
However, joining and modification by a conventional friction stir involves problems. The substance to be process stirred by the tool is discharged by rotation of the tool, and a groove-like defect is produced in a joint or a modified area as if it were subjected to cutting work. In addition, when the rotating tool is pulled out of an aluminum cast or the like after the completion of joining or modification by the friction stir, a concave, designated as so-called key hole, in line with the shape of the tip of the tool can be produced. In case of the above-mentioned piston ring, there is a possibility that the blow-by will be caused by this key hole.
As one of methods for solving the above problems, the following technology has been proposed: in an aluminum cast or the like as the substrate, an area where the ring groove is to be formed is modified by the friction stir to form a homogenized metal structure; and thereafter, the ring groove is formed by machining and then alumite treatment is subjected to the area. (Refer to Document 5 (Japanese Patent Laid-open No. 2007-64129), for example). In the technology disclosed in Document 5, because the ring groove is formed by machining after a friction stir processing, any groove-like defect and key hole formed on the surface of the modified area during the machining is removed.
However, this technology also involves a problem. To completely eliminate key holes from the ring groove by machining, an outside diameter of a tool used in the friction stir must be larger than the groove width of the ring groove. Under these conditions, it is difficult to secure sufficient width and depth of a modified area. In addition, it is required to use a tool of small diameter and this shortens the life of the tool. To ensure a large width of a modified area without regard for the width of the ring groove, the following method can be used: a dolly block is attached to a piston; and the friction stir processing is completed with this dolly block and then the dolly block is removed from the piston. (Refer to Document 6 (Japanese Patent No. 3295376), for example.) In this method, the dolly block must be prepared for each piston and this increases the production cost.
The present invention has been made with the above situation taken into account. It is an object of the present invention to provide an apparatus for a friction stir and a friction stir processing wherein the occurrence of a surface defect in an area processed by the friction stir can be suppressed.